The invention relates to nucleated casting systems and associated methods for forming the casting. In particular, the invention relates to nucleated cast systems and methods that comprise the addition of powders to a casting.
Metals, such as iron- (Fe), nickel- (Ni), titanium- (Ti), and cobalt- (Co) based alloys, are often used in turbine component applications, in which fine-grained microstructures, homogeneity, and essentially defect-free compositions are desired. Problems in superalloy castings and ingots are undesirable as the costs associated with superalloy formation are high, and results of these problems, especially in ingots formed into turbine components are undesirable. Conventional systems for producing castings have attempted to reduce the amount of impurities, contaminants, and other constituents, which may produce undesirable consequences in a casting made from the casting.
Casting to form articles (hereinafter xe2x80x9ccastingsxe2x80x9d) may include at least a step of electroslag refining (ESR) (such as disclosed in U.S. Pat. Nos. 5,160,532; 5,310,165; 5,325,906; 5,332,197; 5,348,566; 5,366,206; 5,472,177; 5,480,097; 5,769,151; 5,809,057; and 5,810,066, all of which are assigned to the Assignee of the instant invention). Other metallurgical methods, such as, but not limited to, refining and mechanical working, may be combined with ESR to further refine and form the casting to reduce the amount of impurities, contaminants, and other constituents. While the metal produced by such a sequence is useful and the metal product itself is valuable, the processing is quite expensive and time-consuming. Further, the processing and refining of relatively large bodies of metal, such as superalloys, is often accompanied by problems, for example problems in achieving homogeneous, defect-free structure.
One such problem that often arises in superalloy casting comprises controlling the grain size and other microstructure of the refined metals during nucleation and solidification from a liquid to a solid. Further, problems of alloy or ingredient segregation also occur as processing is performed on large bodies of metal. Problems may arise during some electroslag refining processing operations. For example, a conventional electroslag refining method typically uses a refining vessel that contains a slag refining layer floating on a layer of molten refined metal. An ingot of unrefined metal is generally used as a consumable electrode and is lowered into the vessel to make contact with the molten electroslag layer. An electric current is passed through the slag layer to the ingot and causes surface melting at the interface between the ingot and the slag layer. As the ingot is melted, oxide inclusions or impurities are exposed to the slag and removed at the contact point between the ingot and the slag. Droplets of refined metal are formed, and these droplets pass through the slag and are collected in a pool of molten refined metal beneath the slag. The electroslag refining apparatus may be dependent on a relationship between the individual method parameters, such as, but not limited to, an intensity of the refined current, specific heat input, and melting rate. This relationship involves undesirable interdependence between the rate of electroslag refining of the metal, metal ingot temperature, and rate at which the refined molten metal is cooled, all of which may result in poor metallurgical structure in the resultant casting.
Another problem that may be associated with conventional electroslag refining processing comprises the formation of a relatively deep metal pool in an electroslag crucible. A deep melt pool may cause a varied degree of ingredient macrosegregation in the metal that leads to a less desirable microstructure, such as a microstructure that is not a fine-grained microstructure, or segregation of the elemental species so as to form an inhomogeneous structure. A subsequent operation has been proposed in combination with the electroslag refining method to overcome this deep melt pool problem. This subsequent processing may be vacuum arc remelting (VAR). Vacuum arc remelting is initiated when an ingot is processed by vacuum arc steps to produce a relatively shallow melt pool, whereby an improved microstructure, which may also possess a lower hydrogen content, is produced. Following the vacuum arc refining method, the resulting ingot is then mechanically worked to yield a metal stock having a desirable fine-grained microstructure. Such mechanical working may involve a combination of steps of forging and drawing. This thermo-mechanical processing requires large, expensive equipment, as well as costly amounts of energy input.
An attempt to provide a desirable casting microstructure has been proposed in U.S. Pat. No. 5,381,847, in which a vertical casting method attempts to control grain microstructure by controlling dendritic growth. The method may be able to provide a useable microstructure for some casting applications. However, the vertical casting method does not control the source metal contents, including but not limited to impurities, oxides, and other undesirable constituents. Further, the vertical casting operation forms a relatively deep liquidus portion in the mold, in which the liquidus portion is slow to solidify due to slow metal nuclei formation therein. The slow nuclei formation slows the casting operation, and may also may adversely impact a casting""s microstructure and characteristics.
Therefore, a need exists to provide metal casting methods and systems that enhance nuclei formation, produce a casting with a relatively homogeneous, fine-grained microstructure, and that can be supplied with a clean metal source. Further, a need exists to provide a methods and systems that produce a casting with a relatively homogeneous, fine-grained microstructure. Further, a need exists to provide methods and systems that produce a casting that is essentially free of oxides, for turbine component applications.
An aspect of the invention sets forth nucleated casting systems and methods that comprise the addition of powders into a liquidus portion of the casting. The casting system forms a casting comprising a liquidus portion that receives the refined liquid metal and a solidified portion, the casting further comprising a fine-grain, homogeneous microstructure that is essentially oxide- and sulfide-free and metal, the refined liquid metal having oxides and sulfides refined out of the metal; a solid metal particle addition system that adds solid metal particles to a surface of the liquidus portion of the casting; and a nucleated casting system for forming the casting. The solid metal particle addition system adds solid metal particles that serve as nucleation centers during solidification of the casting.
A further aspect of the invention comprises a casting method with solid metal particle addition for forming a casting. The casting comprises a liquidus portion that receives the refined liquid metal and a solidified portion. The casting further comprises a fine-grain, homogeneous microstructure that is essentially oxide- and sulfide-free and segregation defect free. The casting method comprises providing a source of refined liquid metal, supplying the refined liquid metal to a mold; adding solid metal particles to the casting; forming a casting by nucleated casting, the casting comprising a liquidus portion and a solidified portion. The solid metal particles serve as nucleation centers during solidification.
These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, where like parts are designated by like reference characters throughout the drawings, disclose embodiments of the invention.